The present invention relates to a circuit that monitors a power supply voltage supplied to a load through a constant voltage circuit including a ripple filter.
The constant voltage circuit is used in order to supply a constant power supply voltage (load voltage) to a load even if the voltage input from the voltage source changes due to various factors. In order for the constant voltage circuit to operate normally, the input voltage should be higher than the load voltage (output voltage).
In each product, there is a variation of a few percent in the input and output voltage characteristics of the constant voltage circuit. Accordingly, in order to operate the constant voltage circuit normally even if the output voltage is at the upper limit of the range of variation, the input voltage value needs to be set high in order to have a margin. This means that power consumption in the constant voltage circuit is increased and accordingly efficiency is lowered.
For example, the constant voltage supply function may also be obtained by using the ripple filter disclosed in Patent References 1 to 3. Since the ripple filter is configured to remove a high-frequency fluctuation component included in the voltage supplied from the voltage source, it is possible to obtain approximately the same output voltage as the input voltage. Since the above-described high input voltage setting is not necessary, power consumption is low.
(Patent Reference 1) JP-A-6-284715
(Patent Reference 2) JP-B-62-31370
(Patent Reference 3) JP-B-52-40740
When another load, through which a large current flows, is connected to the voltage source, the operation of the load may cause a drop in the input voltage supplied to the ripple filter. Since the ripple filter is not configured to eliminate a variation in the electric potential due to the drop in input voltage, the output voltage becomes a voltage given by removing a ripple component from the input voltage after the drop.
That is, the output voltage from the ripple filter may be lower than the power supply voltage that a load, connected to the ripple filter, requires. In this case, the load cannot perform a predetermined operation.